1. Field of the Invention
This invention relates to an active carbon pellet prepared by extruding activated lignocellulosic-based carbon with a binder material. More particularly, the invention relates to an improved active carbon pellet characterized by low pellet void volume and low dust attrition.
2. Description of the Prior Art
Granular carbons and carbon pellets are typically used in columns or beds for gas and vapor systems as well as for processing a number of liquids. Such carbons have been used in canisters in automobiles through which gasoline tank and carburetor vapors are directed prior to release to the environment. To qualify for this application, a carbon must possess sufficient mechanical strength to withstand the abrasion incident to continued use.
There generally is a direct correlation between the mechanical strength of the granular activated carbon product and the mechanical strength of its precursor raw material. Thus, coal-based active carbon generally exhibits a high mechanical strength and density; whereas, lignocellulosic-based active carbons, derived from a much "softer" precursor relative to coal, generally exhibit low mechanical strengths and densities.
Also, gas-adsorbing carbons should be as dense as is consistent with high adsorption capacity so as not to require a large space for the adsorber. The development of high adsorption capacity during thermal activation, however, is accompanied by a loss of mechanical strength and density; therefore, some compromise is required in selecting the degree to which the activation is conducted. So, with lignocellulosic precursors (or, for lignocellulosic-based active carbons), the problem is compounded.
Several approaches have been taken to address the problem of low mechanical strength and density of lignocellulosic-based active carbons. In U.S. Pat. No. 3,864,277, Kovach emphasizes the binder additive in teaching the phosphoric acid activation of wood, straw, or low-rank brown coals in the presence of a carbonaceous binder material such as lignosulfonates and polyvinyl alcohol, followed by forming solid granular shaped particles from the mixture, and heat-treating at less than 650.degree. C. to give a granular product having a ball-pan hardness of greater than 85%. Given the teaching of Kovach and employing the knowledge of the relationship of precursor mechanical strength and density with those characteristics of the active carbon product, MacDowall (in U.S. Pat. No. 5,162,286) teaches increasing lignocellulosic-based active carbon density by the use of young carbonaceous vegetable products high (&gt;30%) in natural binding agent, such as nut shell, fruit stone, almond shell, and coconut shell, as precursors for treatment with phosphoric acid followed by carbonization.
A third approach, which relates to the present invention, is taught by McCue et al. in U.S. Pat. No. 4,677,086. To achieve, in a wood-based active carbon, the mechanical strength and product density approaching that achieved with coal-based products, McCue et al. teach extruding an active wood-based carbon with bentonite clay, followed by calcining the extruded active carbon/clay pellets. This technology has been the basis for the commercial products NUCHAR.RTM. BAX-950 and NUCHAR.RTM. BAX-1100 marketed by Westvaco Corporation.
Carbons of suitable mechanical strength and density for use in an evaporative emission control device (automotive canister) for adsorbing gasoline vapors preferably also exhibit a butane working capacity of about 10 to about 17 g/100 cc and an apparent density from about 0.25 to about 0.40 g/cc.
In addition to gas column (or, packed bed) requirements for high mechanical strength and high density, it is also desirable to reduce the bed void volume in order to maximize the carbon content of the bed, and subsequently maximize the adsorptive capacity. This is primarily determined by the shape of the granular or pelleted carbon. In fact, because of the irregular shape of granular carbon, more regularly shaped carbon pellets are preferred for their better "packing." However, as a result of uneven cutting of the extrudate to form the pellets, the pellets are, in fact, irregularly shaped, and fissures and cavities often appear along the pellet surface. This creates two problems. The resulting irregularities in shape prevent optimization of bed (or column) packing and detract from maximizing the carbon content for a given pellet volume. In addition, the surface irregularities are often removed from the pellet due to abrasion. These material losses, in addition to debris caused by cutting the pellets to size, present another problem: dust. Typically, dusting due to abrasion, or dust attrition, may be retarded or precluded by spraying a coating on the surface of the pellet. Invariably, this remedy is at the expense of butane working capacity; thereby providing another trade-off for the working life of the active carbon material.
Besides having a product which may appear to disintegrate, attrited dust in a packed bed, such as a column or an automotive canister can fill the bed voids to create high pressure drops and impede the flow-through of vapors to be treated. A particular problem in the automotive application is concern that the dust will act to interfere with various sensing devices connected to the canister to monitor performance, resulting either in false readings or in failure of the sensing devices altogether.
Therefore, an object of this invention is to provide an improved lignocellulosic-based activated carbon pellet of a smoother surface and more uniform shape which provides optimal bed packing, exhibits increased density, and is less susceptible to dust attrition. An additional object of this invention is to provide an improved method of manufacture of such activated carbon pellet.